The importance of estimating ice sheet mass balance

The mass balance of a glacier or ice sheet is the net balance between the mass gained by snow deposition, and the loss of mass by melting (either at the glacier surface or under the floating ice shelves or ice tongues) and calving (production of icebergs). A negative mass balance means that a glacier is losing mass, and, for grounded glaciers and ice sheets, this mass loss directly contributes to sea level rise (the melting of floating ice shelves and ice tongues does not contribute to sea level rise, because of the lower density of ice as compared to water, which determines the floating portion of the ice). This is one of the reasons why it is important, under a warming climate, to have accurate estimates of the mass balance of glaciers and ice sheets.

How is the mass balance estimated?

Past mass balance rates can be estimated from ice core data, although the proper dating of the samples is challenging. For the deeper parts of the ice core (representing the older data), the dating requires modelling the ice sheet dynamics.

For the large ice sheets of Antarctica and Greenland, the estimate of present mass balance is only possible using remote sensing (satellite or airborne) techniques, though these need to be calibrated and validated against measurements done on the glacier surface.

For predicting future mass balance, under different scenarios of climate change, it is necessary to use models of the dynamics and thermal regime of the glaciers and ice sheets. These models have to be integrated with climate models (that provide the information on accumulation and melting at the glacier surface) and oceanic models (which provide the interaction between the ice sheets and the ocean).

ISMASS Expert Group

The Expert Group on Ice Sheet Mass Balance and Sea Level (ISMASS) is co-sponsored by the Scientific Committee on Antarctic Research, the International Arctic Science Committee (IASC), and the WCRP Climate and Cryosphere Project.The goals of ISMASS are to promote the research on the estimation of the mass balance of ice sheets and its contribution to sea level, to facilitate the coordination among the different international efforts focused on this field of research, to propose directions for future research in this area, to integrate the observations and modelling efforts, as well as the distribution and archiving of the corresponding data, to attract a new generation of scientists into this field of research, and to contribute to the diffusion, to society and policy makers, of the current scientific knowledge and the main achievements in this field of science. Further details on the goals of ISMASS can be found in the new Terms of Reference.

A review of ice sheets in a 1.5°C warmer world (a new recent climate mitigation target highlighted by the Intergovernmental Panel on Climate Change) suggests that current predictions of future changes in ice-sheet mass balance are questionable due to incomplete understanding of atmospheric circulation changes around Greenland and ocean circulation around Antarctica. However, the study concludes it is important to limit global warming by 2100 to 1.5°C to maximise the chance of avoiding so-called tipping points that would dramatically accelerate mass loss.

Today, sea level rises at a pace of around 4mm per year and is accelerating. The major contributions to this rise are due to ocean expansion, melting glaciers and – becoming increasingly important – melting of the Greenland and Antarctic ice sheets. Those ice sheets are commonly considered as the ‘sleeping giants’ on Earth, as together they can potentially lead to sea-level rise by more than 70 metres over thousands of years.

Despite improved observations and computer model simulations since the IPCC’s Fifth Assessment Report (AR5) published in 2013, the review highlights some lingering key gaps in knowledge regarding climate forcing of the ice sheets and their resulting responses. On longer time scales, both ice sheets may reach tipping points above around a 1.5-2.0°C limit, leading to irreversible mass loss. For Greenland this is due to increased melting of the ice sheet, while for Antarctica this is due to marine instabilities of certain sectors of the ice sheet, in particular the West Antarctica ice sheet. This would lead to multi-metre sea level rise on centennial to millennial time scales, which has existential consequences for small island states and coastal cities across the world.

Lead author Prof. Frank Pattyn said “Limiting global atmospheric warming to 1.5°C will avoid short and long-term surprises coming from both ice sheets and significantly reduce adaptation cost when global sea-level rise is limited and not catastrophic”.

2017 Updates from ISMASS

ISMASS Workshop addresses impact of 1.5°C global warming on Greenland & Antarctic ice sheetsAn Ice Sheet Mass Balance and Sea Level (ISMASS) workshop was held in Brussels from 11‐13 January 2017 with the purpose of preparing a review paper on the contribution of Greenland and Antarctic ice sheets to future sea level under a 1.5‐degreeC warmer climate (in line with the Paris agreement). This workshop was funded by SCAR and CliC. Discussions at the meeting led to the conclusion that the review paper would address the following:Forcing (non‐linearities, SMB, circulation changes, feedbacks...), Advances in understanding processes‐uncertainties since IPCC Assessment Report 5 (calving, GIA, basal processes...), Expected centennial response for Greenland and Antarctica, and Expected long‐term response (including commitment). The resulting review paper is planned to feed into the IPCC Special Report on 1.5 degrees.Participants at the meeting were Frank Pattyn (organizer), Lionel Favier, Gael Durand, Catherine Ritz co‐organiser and chair of ISMASS), Xavier Fettweis, Edward Hanna, Michiel van den Broeke, Heiko Goelzer, Xylar Asay‐Davis, Alexander Robinson, Tony Payne, Helene Seroussi and Sophie Nowicki. Rob DeConto joined by Skype.Other 2017 HighlightsCatherine Ritz gave presentations on the above activity at: (1) 2017 European Geosciences Union meeting; and (2) WCRP/IOC conference on “Regional Sea Level Changes and Coastal Impacts” in New York in July 2017.

2018 PublicationA review paper arising from the January 2017 Brussels workshop is in preparation, intended for submission to Nature Climate Change.

Plans of ISMASS for 2018 and beyond

‐There was an ISMASS related session during the Open Conference in DAVOS in 2018. CR5: “Interactions between ice mass balance, the solid Earth, and sea‐level changes” (C. Ritz convener)‐ ISMASS organized a one‐day workshop on 15 June 2018 as a side meeting of the above Open Conference. Title: “Update on mass balance of Greenland and Antarctica (linkages between data and models)”. A review article should be an outcome of this workshop (Contact Edward Hanna)‐ The review article on “Contribution of Greenland and Antarctic ice sheets to future sea level under a 1.5 degree warmer climate” will be finalised and submitted during spring 2018 (contact F. Pattyn, C. Ritz) ‐ ISMASS will continue to endorse the ice‐sheetocean model intercomparisons by helping organizing workshops related to this issue (e.g., MISOMIP).‐ An ISMASS Steering Committee Meeting was held on 17 June (at the Davos SCAR open conference).